Complement plays an essential role in the organism's defence against infectious agents and in the inflammatory process. It represents a helper system for immunity, in particular for the humoral response and for the innate response.
Complement comprises a set of approximately 30 plasma and sometimes membrane proteins, synthesized essentially by the liver and the macrophages and which can be activated by proteolytic cascades. It comprises both plasma proteins, numerous different surface cell receptors, some present on inflammatory cells and others on immune system cells, and also regulatory membrane proteins which protect the host cells against self-attack. The complement plasma proteins operate either as enzymes, or as binding proteins, or as regulators (inhibitors or activators).
There are 3 pathways for triggering the complement cascade: the classical pathway triggered by antibodies, the alternative pathway triggered by bacterial substances in the absence of antibodies, and the pathway dependent on lectins which recognize certain bacterial polysaccharides. The latter two pathways are involved in the innate response and are extremely important in the host's defence against bacterial infections.
Complement factor B is a component of the alternative complement activation pathway. It circulates in the blood as a protein composed of a single polypeptide chain with a molecular weight of 93 kDa. During activation of the alternative pathway, factor B is cleaved by complement factor D, resulting in an inactive Ba chain (30 kDa) and in the Bb catalytic subunit (63 kDa). The Bb active subunit is a serine protease which associates with C3b to form the C3 convertase of the alternative pathway (C3bBb). The Bb subunit is involved in the proliferation of pre-activated B lymphocytes, while the Ba chain inhibits their proliferation.
Factor B preparations would be of use in the treatment of factor B deficiencies, but for the moment, no preparation of this type is commercially available.